How does light determine whether it acts as a particle or a wave?

Light doesn't need to decide. It simply behaves both ways at the same time.
If you create an experiment that can detect particles and run light through it, the experiment detects particles.
If you build an experiment that can detect waves and run light through it, the experiment detects waves.
Wierd but true.

Neptune is the eighth planet from the sun. it was the first planet to get its existence predicted by  mathematical calculations  before it was actually seen through a telescope on sept. 23, 1846. irregularities in the orbit of  uranus  led french astronomer alexis bouvard to suggest that the gravitational pull from another celestial body might be responsible. german astronomer johann galle then relied on subsequent calculations to   spot neptune via telescope. previously, astronomer galileo galilei sketched the planet, but he mistook it for a star due to its slow motion. in accordance with all the other planets seen in the sky, this new world was given a name from greek and roman mythology — neptune, the roman god of the sea.